Manual phalangeal curvature and its relationship to positional behavior in anthropoids

The presence of relatively long and curved manual phalanges in both extinct and extant anthropoid species has been used in the past as evidence of suspensory positional behavior. However, the relationships between phalangeal length, curvature, and positional behavior are incompletely understood. Some models predict that both phalangeal length and body mass will have an effect on phalangeal curvature out of biomechanical necessity. A better understanding of these relationships can help us to reconstruct the positional behavior of extinct relatives, including the precursors of our unique bipedalism. Phalangeal measurements and body mass estimations were collected from a broad sample of living anthropoid species. As one would expect, phalangeal length is tightly correlated with body mass. Phalangeal curvature (degrees of arc) is strongly correlated with length across the entire sample, but more weakly within taxonomic groups. Curvature is weakly correlated with body mass across the entire sample, and individuals are tightly clustered based on positional behavior. Phalangeal curvature and body mass together are able to predict the dominant positional behavior of an individual with 80% accuracy. Intra-group variation in curvature is still unexplained, but there is evidence for an ontogenetic remodeling component to this trait that may reflect individual variations in behavior. The hand of the recently described Ardipithecus ramidus, interpreted as an above-branch walker, clusters with animals with a more significant degree of suspension.